A conventional vacuum operated booster is disclosed in U.S. Pat. No. 4,587,885. This booster comprises a housing having an opening at one end, a power piston extending through the opening, a diaphragm that divides the inside of the housing into a constant pressure chamber and a variable pressure chamber. A valve plunger is fitted in a small hole or opening formed in the power piston so as to be slidable, and a control valve is disposed in the power piston which can hermetically slide in the opening. The outer fringe or edge of the diaphragm is hermetically held to the housing, while the inner fringe or edge hermetically fits over the outer periphery of the power piston. The valve plunger is connected to a pushrod which interlocks with the brake pedal of an automobile. When the booster is not in operation, the control valve bears against the rear end of the valve plunger. The booster further includes a control valve mechanism, an output rod having a rear end portion, and a reaction disk that is mounted between the rear end portion of the output rod and the power piston in a large hole or opening formed in the power piston, the large hole being in communication with the small hole or opening. The rear end portion of the output rod is fitted in the large hole or opening so as to be slidable therein. The control valve mechanism connects the variable pressure chamber either with the constant pressure chamber or with the atmosphere, depending on whether or not the brake pedal is depressed. To limit axial movement of the output rod within the large hole or opening in the power piston, a retainer is disposed in the constant pressure chamber. One end of the retainer is anchored to a sleeve that is fixed to the power piston, while the other end can come into contact with the end surface of the output rod which is on the side of the constant pressure chamber.
When the conventional vacuum booster constructed as described above is not in operation, a given or predetermined space is formed between a master cylinder piston and the front end of the output rod to prevent the brakes from dragging. The retainer simply acts to prevent the output rod from coming off and does not press the output rod against the piston. Therefore, when the booster is not in operation, the output rod tilts and causes a gap to open between the reaction disk and the end surface of the power piston. At the beginning of the operation, the gap between the control valve and the seat portion of the power piston is closed causing air to flow into the variable pressure chamber. At this time, the pressure inside the variable pressure chamber must be different from the pressure inside the constant pressure chamber. However, air flows from the variable pressure chamber into the constant pressure chamber through the aforementioned gap between the reaction plate and the end surface of the piston. As a result of the air flow, the booster does not work quickly at the beginning of the booster operation sequence.